1. Field of the Invention
The present invention relates to thermographic machines for relief printing.
2. Description of the Prior Art
Thermographic is a known process. It enables, from a typographic printing, offset or other, to obtain a relief or raised printing.
The transformation in relief is simple and consists of sprinkling a freshly printed sheet of paper while the ink is still wet, with a powder having the ability to melt under heat. After fusion due to heat, a film in relief is created.
The wet ink retains only the powder, and the excess is continuously recycled. The powder printed paper then goes through a tunnel-shaped furnace. At the exit, a stream of fresh air cools the paper and instantaneously congeals the viscous film in relief to prevent the sheets of paper from sticking to each other.
The relief automatic transformation is operated in the following manner: The paper, exiting from the printing press, is received directly on the carrying bands and goes successfully under the powdering assembly, inside the tunnel-shaped furnace and onto a last conveyor to cool before going into a receiving tub.
The current machines in use present the following shortcomings and faults: They do not enable to change into relief a printed paper comprising several settings and to simultaneously cut it. The printer has to take the printed paper in relief and cut it with another machine called a slitter. This proceeding brings additional expensive manipulations. Also, the prior art powdering assemblies are equipped with powder vacuum assemblies which are not reversible from one side to the other of the machine. Thermographic machines are often moved from one printing press to another in the same shop, and the placement of the controls for the different printng presses differ. Depending upon the type of printing press, the lack of reversibility of a powder vacuum chamber may result in an operator of the printing press having difficulty watching the operation of the powder vacuum chamber.
Another inconvenience is found in the manner normally used in heating the coat of powder destined to become the film foring the relief. In the prior art, the heating is obtained in a tunnel-shaped furnace, electric or gas, in which passes the powdered printed paper.
With this principle, the paper or support for the powder must progressively reach the temperature of fusion of the powder, of about 90 to 100 degrees centigrade. The length of warming time of the paper, particularly in the case of high gram weight paper, requires furnaces of great length to obtain rapid rates, such as 6,000 to 8,000 pieces per hour. The hot air obtained by convection inside the furnace is about 300 to 450 degrees centigrade. This air is not pulsated onto the printed paper in the prior art convection machine.
This procedure results in a number of problems. Most tunnel-shaped furnaces are very large. It is difficult to treat heavy paper without risk of damage. Cooling of the printing paper, after fusion, is difficult and costly because of the high expense of energy necessary to remove the heat absorbed by the paper during its heating. The paper also dehydrates due to the heat, which is detrimental to its dimensional stability.